The present invention relates to curtain assemblies and more particularly to furnace curtain assemblies effective to substantially block gas flows in heat treating furnaces.
In many instances, such as in connection with the heat treating of ferrous parts, it is necessary to establish different gaseous atmospheres in separate zones of a heat treating furnace, or the like. For example, in the neutral hardening or annealing of such parts, different gaseous atmospheres are established in the preheat, hot and cooling zones of a furnace. An example of such a furnace zoning is found in British Patent Application No. 81-28392 and U.S. Pat. No. 4,294,395, the latter being directed to a process for brazing parts in a nitrogen based atmosphere and which is assigned to the assignee of the present invention. Typically, zoning of such furnaces is achieved by the use of sliding doors which are raised and lowered to enable the passage of parts being treated, but which are cumbersome and inefficient in furnaces through which parts are continuously passed as, for example, on a conveyor belt or the like. As the sliding doors of such furnaces are frequently opened and closed, it is difficult to maintain zones of separate gaseous atmospheres. The use of gas barriers such as inert gas flows or endothermic or exothermic gas flows has been proposed, but such gases tend to mix with a particular gaseous atmosphere in zones on either side of such "barrier".
It is also known to utilize relatively heavy but somewhat flexible curtains, typically at furnace entrances and exits to reduce the flow of ambient atmosphere into a furnace or the outward flow of furnace atmosphere adjacent to such inlet or outlet. One problem with the use of such curtains is that as the same pass over parts being translated through the furnace for heat treatment, etc., the curtains are deflected and expose considerable areas of the furnace of one zone to another zone and are thus relatively ineffective as gas barriers. If the weight of such curtains is increased to minimize such deflection, relatively light but large parts undergoing heat treatment, such as carburetor housings, etc., may be knocked over which in turn may lead to undesirable flows of brazing fluxes, etc. and the ineffective brazing of particular joints.
One solution to some of the foregoing problems is the use of curtains formed of a plurality of cords of heat resistant material. These curtains are formed by clamping mounting plates above and below a plurality of cords laid adjacent to one another and up to several layers deep. In this manner, a pair of curtain sections, each formed from many such cords, hang freely from the edge of the mounting plates. As parts are passed through a heat treating furnace, etc., in which such curtain assemblies are disposed, individual cord members will be deflected, but as only those cord members which contact the parts are deflected, a relatively effective barrier to gas flows between zones of the furnace is established. One problem with such curtains is that by clamping mounting plates directly onto cord members, the edges of the mounting plates, as well as the bolts securing such plates to one another, tend to wear and cut the cord members. In some instances, such cut cords fall on parts being heat treated which requires an operator to eventually remove such cords. In addition, the ends of cord members wear under furnace conditions which also mandates replacement thereof. Heat resistant cordage such as is sold under the trademark "Refrasil," is expensive and replacement of the same, as will be described below, is relatively time consuming and/or inconvenient. By employing mounting plates, as mentioned above, the curtain assembly cannot be readily replaced or stored independently of the mounting hardware. Thus, in order to replace a worn curtain assembly, the clamps securing the mounting assembly are released which results in a mass of cord members, which if acceptable for reuse, must be realigned in order to form a rebuilt curtain assembly. Alternately, spare curtain assemblies must be stored attached to or secured to the mounting hardware, and as the same may be anywhere between 18 inches to 8 feet in length, such storage or inventory of mounted curtain assemblies requires considerable space and is frequently impractical.
Thus, there is a clear need for furnace curtain assemblies which effectively block gas flows and yet are readily replaceable when worn but which are not excessively costly and may be easily stored when not in use.